Statistical analysis Statistical analysis will be performed by the SPSS program for Windows, version 10.1 (SPSS Inc, Chicago, IL). Continuous variables will be presented as mean ± standard deviation, and categorical variables as absolute numbers and/or percentages. Data will be checked for normality before statistical analysis. Normally-distributed continuous variables will be compared using the unpaired t test, whereas the Mann-Whitney U test will be used for those variables that are not normally distributed. Categorical variables will be analyzed using either the chi-square test or Fischer's exact test. Comparison between and within groups will be made using two-way analysis of variance for repeated measures. Comparisons will be considered significant if p<0.05.